Move away from super.tap usage in user assignment creation

- currently we are relying on a cached check if the user
  is already assigned to be cached before the work is
  done to assign the user. Then after that user is added
  we have a check to see that the user isn't already assigned
  based on the cache only to retain that knowledge.
  It would be better to merely call this on success if
  the user is actually assigned during the service execute,
  which is what we are trying to measure here.
  Changing this could also better insulate us from future
  regressions as it is more declaritive about when and
  why the work happens.

- see https://gitlab.com/gitlab-org/gitlab/-/issues/508955

ee/app/services/gitlab_subscriptions/user_add_on_assignments/base_create_service.rb

@@ -38,9 +38,11 @@ def execute
           Rails.cache.delete(user.duo_pro_cache_key_formatted)
 
           log_event('User AddOn assignment created')
-
-          ServiceResponse.success
         end
+
+        after_success_hook
+
+        ServiceResponse.success
       rescue NoSeatsAvailableError => error
         Gitlab::ErrorTracking.log_exception(
           error, base_log_params.merge({ message: 'User AddOn assignment creation failed' })
@@ -53,6 +55,10 @@ def execute
 
       attr_reader :add_on_purchase, :user
 
+      def after_success_hook
+        # overridden in inheriting classes.
+      end
+
       def validate
         return ERROR_NO_SEATS_AVAILABLE unless seats_available?
 

ee/app/services/gitlab_subscriptions/user_add_on_assignments/saas/create_service.rb

@@ -4,16 +4,15 @@ module GitlabSubscriptions
   module UserAddOnAssignments
     module Saas
       class CreateService < ::GitlabSubscriptions::UserAddOnAssignments::Saas::CreateWithoutNotificationService
-        def execute
-          super.tap do |response|
-            create_iterable_trigger if should_trigger_duo_iterable? response
-          end
-        end
+        extend ::Gitlab::Utils::Override
 
         private
 
-        def should_trigger_duo_iterable?(response)
-          response.success? && duo_pro_or_enterprise? && !user_already_assigned?
+        override :after_success_hook
+        def after_success_hook
+          super
+
+          create_iterable_trigger if duo_pro_or_enterprise?
         end
 
         def duo_pro_or_enterprise?

ee/app/services/gitlab_subscriptions/user_add_on_assignments/saas/create_without_notification_service.rb

@@ -8,8 +8,6 @@ class CreateWithoutNotificationService < ::GitlabSubscriptions::UserAddOnAssignm
 
         private
 
-        attr_reader :add_on_purchase, :user
-
         def eligible_for_gitlab_duo_pro_seat?
           namespace.eligible_for_gitlab_duo_pro_seat?(user)
         end

ee/app/services/gitlab_subscriptions/user_add_on_assignments/self_managed/create_service.rb

@@ -5,16 +5,16 @@ module UserAddOnAssignments
     module SelfManaged
       class CreateService < ::GitlabSubscriptions::UserAddOnAssignments::BaseCreateService
         include Gitlab::Utils::StrongMemoize
-
-        def execute
-          super.tap do |response|
-            send_duo_seat_assignment_email if should_send_duo_seat_assignment_email? response
-          end
-        end
+        extend ::Gitlab::Utils::Override
 
         private
 
-        attr_reader :add_on_purchase, :user
+        override :after_success_hook
+        def after_success_hook
+          super
+
+          send_duo_seat_assignment_email if Feature.enabled?(:duo_seat_assignment_email_for_sm, :instance)
+        end
 
         def eligible_for_gitlab_duo_pro_seat?
           user.eligible_for_self_managed_gitlab_duo_pro?
@@ -25,12 +25,6 @@ def send_duo_seat_assignment_email
           DuoSeatAssignmentMailer.duo_pro_email(user).deliver_later if add_on_purchase.add_on.code_suggestions?
           DuoSeatAssignmentMailer.duo_enterprise_email(user).deliver_later if add_on_purchase.add_on.duo_enterprise?
         end
-
-        def should_send_duo_seat_assignment_email?(response)
-          Feature.enabled?(:duo_seat_assignment_email_for_sm, :instance) &&
-            !user_already_assigned? &&
-            response.success?
-        end
       end
     end
   end